Imaging device and imaging system

ABSTRACT

An imaging device includes a plurality of pixels each including a plurality of avalanche photodiodes, a setting unit configured to set the plurality of avalanche photodiodes to an active state or an inactive state separately, and a counter circuit that counts and outputs number of photons determined by the avalanche photodiode(s) set to the active state out of the plurality of avalanche photodiodes, wherein the imaging device is configured to change the number of avalanche photodiodes set to the active state out of the plurality of avalanche photodiodes in accordance with brightness of an object.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to an imaging device and an imaging system

Description of the Related Art

In recent years, application of semiconductor devices that are able todetect a faint light in a level of a single photon is expected in a widerange of fields. In particular, a use of a photodetector element inwhich a signal corresponding to a single photon is larger than a noiseat signal readout allows for so called photon-counting in which thebrightness of an input light, which has been conventionally handled ascontinuous values, is accurately counted as a discrete value of thenumber of photons.

An example of photodetector elements that realize photon-counting may bean avalanche photodiode (hereafter, also referred to as “APD”). An APDcan amplify the amount of signal charges excited by photons by aroundseveral times to million times by using an avalanche amplificationphenomenon caused by an intense electric field induced at a p-n junctionof a semiconductor. By utilizing a high gain of the avalancheamplification phenomenon, it is possible to amplify a faint light signalto be sufficiently greater than a readout noise and realize brightnessresolution in the level of a single photon.

Japanese Patent Application Laid-Open No. 2013-020972 discloses animaging device in which single-photon avalanche diodes (hereafter, alsoreferred to as “SPAD”) that cause the APD to operate in a Geiger modeare arranged in a two-dimensional manner. The imaging device disclosedin Japanese Patent Application Laid-Open No. 2013-020972 seeksimprovement of the image quality by not using SPAD pixels which do notcontribute to an optical signal or SPAD pixels which have a large darkcount rate (DCR) noise representing noise in APDs.

While the SPAD is superior in detection of a faint light, however, thedetection frequency of photons will be too high in a bright situationsuch as under a day light even when the light amount is reduced by usingan aperture mechanism. Thus, in an imaging device with SPADs, it isdifficult to support both capturing of a dark scene and capturing of abright scene by the same pixels.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an imaging device withSPADs that is able to easily control photoelectric change gain in apixel and an imaging system using the imaging device.

According to an aspect of the present invention, there is provided animaging device including a plurality of pixels each including aplurality of avalanche photodiodes, a setting unit configured toseparately set the plurality of avalanche photodiodes to an active stateor an inactive state, and a counter circuit that counts and outputsnumber of photons determined by the avalanche photodiode(s) set to theactive state out of the plurality of avalanche photodiodes, wherein theimaging device is configured to change the number of avalanchephotodiodes set to the active state out of the plurality of avalanchephotodiodes in accordance with brightness of an object.

According to another aspect of the present invention, there is provideda method of driving an imaging device including a plurality of pixelseach including a plurality of avalanche photodiodes, a setting unitconfigured to set the plurality of avalanche photodiodes to an activestate or an inactive state separately, and a counter circuit that countsand outputs number of photons determined by the avalanche photodiode(s)set to the active state out of the plurality of avalanche photodiodes,the method includes, in the plurality of pixels, setting the number ofavalanche photodiodes set to the active state out of the plurality ofavalanche photodiodes to the same number in accordance with brightnessof an object.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a general configuration of animaging device according to a first embodiment of the present invention.

FIG. 2 is a block diagram illustrating a general configuration of apixel of the imaging device according to the first embodiment of thepresent invention.

FIG. 3 is a circuit diagram illustrating a configuration example of aphotoelectric conversion signal generation unit of the pixel of theimaging device according to the first embodiment of the presentinvention.

FIG. 4 is a cross-sectional view illustrating a general configuration ofavalanche photodiodes of the pixel of the imaging device according tothe first embodiment of the present invention.

FIG. 5 is a graph illustrating an example of a current-to-voltagecharacteristic of an avalanche photodiode.

FIG. 6 is a graph illustrating a relationship between the number ofactually incident photons and the number of counted photons.

FIG. 7A and FIG. 7B are graphs illustrating temporal changes of outputvoltages of inverter circuits and an OR circuit.

FIG. 8 is a top view of the pixel of the imaging device according to thefirst embodiment of the present invention.

FIG. 9 is a circuit diagram illustrating a configuration example of aphotoelectric conversion signal generation unit of the pixel of theimaging device according to a second embodiment of the presentinvention.

FIG. 10A, FIG. 10B, and FIG. 10C are diagrams illustrating a method ofdriving an imaging device according to a third embodiment of the presentinvention.

FIG. 11A, FIG. 11B, FIG. 11C, and FIG. 11D are diagrams illustrating amethod of driving an imaging device according to a fourth embodiment ofthe present invention.

FIG. 12 is a cross-sectional view illustrating a general configurationof an imaging device according to a fifth embodiment of the presentinvention.

FIG. 13 and FIG. 14 are flowcharts illustrating a method of driving animaging device according to a sixth embodiment of the present invention.

FIG. 15 is a block diagram illustrating a general configuration of animaging system according to a seventh embodiment of the presentinvention.

FIG. 16A is a diagram illustrating a configuration example of an imagingsystem according to an eighth embodiment of the present invention.

FIG. 16B is a diagram illustrating a configuration example of a movingunit according to the eighth embodiment of the present invention.

DESCRIPTION OF THE EMBODIMENTS

Preferred embodiments of the present invention will now be described indetail in accordance with the accompanying drawings.

First Embodiment

An imaging device and a method of driving the same according to a firstembodiment of the present invention will be described with reference toFIG. 1 to FIG. 8. FIG. 1 is a block diagram illustrating a generalconfiguration of an imaging device according to the present embodiment.FIG. 2 is a block diagram illustrating a general configuration of apixel of the imaging device according to the present embodiment. FIG. 3is a circuit diagram illustrating a configuration example of aphotoelectric conversion signal generation unit of the pixel of theimaging device according to the present embodiment. FIG. 4 is across-sectional view illustrating a general configuration of avalanchephotodiodes of the pixel of the imaging device according to the presentembodiment. FIG. 5 is a graph illustrating an example of acurrent-to-voltage characteristic of an avalanche photodiode. FIG. 6 isa graph illustrating a relationship between the number of actuallyincident photons and the number of counted photons. FIG. 7A and FIG. 7Bare graphs illustrating temporal change of output voltages of invertercircuits and an OR circuit. FIG. 8 is a top view of the pixel of theimaging device according to the present embodiment.

As illustrated in FIG. 1, the imaging device 100 according to thepresent embodiment includes a pixel region 10, a vertical scanningcircuit 20, a column readout circuit 30, a horizontal scanning circuit40, a control circuit 50, and an output circuit 60.

In the pixel region 10, a plurality of pixels 12 are provided arrangedin a matrix over a plurality of rows by a plurality of columns. On eachof the rows of a pixel array in the pixel region 10, a control signalline 14 is arranged extending in the row direction (the horizontaldirection in FIG. 1). The control signal line 14 is connected torespective pixels 12 aligned in the row direction, which is a signalline common to these pixels 12. Further, on each of the columns of thepixel array in the pixel region 10, a vertical output line 16 isarranged extending in the column direction (the vertical direction inFIG. 1). The vertical output line 16 is connected to respective pixels12 aligned in the column direction, which is a signal line common tothese pixels 12.

The number of pixels 12 forming the pixel region 10 is not limited inparticular. For example, the pixel region 10 may be formed of severalthousand rows by several thousand columns of the pixels 12 as seen in ageneral digital camera. Alternatively, the pixel region 10 may be formedof a plurality of pixels 12 aligned in one row or one column.Alternatively, the pixel region 10 may be formed of a single pixel 12.

The control signal line 14 on each row is connected to the verticalscanning circuit 20. The vertical scanning circuit 20 is a circuit unitthat supplies, to the pixels 12 via the control signal lines 14, controlsignals for driving readout circuits in the pixels 12 when reading outpixel signals from the pixels 12. One end of the vertical output line 16on each column is connected to the column readout circuit 30. Pixelsignals read out from the pixels 12 are input to the column readoutcircuit 30 via the vertical output lines 16. The column readout circuit30 is a circuit unit that performs predetermined signal processing, forexample, signal processing such as an amplification process or ananalog-to-digital (AD) conversion process on the pixel signals read outfrom the pixels 12. The column readout circuit 30 may include adifferential amplifier circuit, a sample-and-hold circuit, an ADconversion circuit, or the like.

The horizontal scanning circuit 40 is a circuit unit that supplies, tothe column readout circuit 30, control signals for transferring thepixel signal processed in the column readout circuit 30 to the outputcircuit 60 sequentially on a column basis. The control circuit 50 is acircuit unit that supplies control signals for controlling operationsand the timings of the operations of the vertical scanning circuit 20,the column readout circuit 30, and the horizontal scanning circuit 40.The output circuit 60 is a circuit unit that is formed of a bufferamplifier, a differential amplifier, or the like and outputs the pixelsignals read out from the column readout circuit 30 to a signalprocessing unit outside the solid state imaging device 100.

FIG. 2 is a block diagram illustrating a general configuration of thepixel 12. As illustrated in FIG. 2, each of the pixels 12 includes aphotoelectric conversion signal generation unit 110, a counter circuit120, and a select circuit 130. The photoelectric conversion signalgeneration unit 110 is connected to the counter circuit 120. The countercircuit 120 is connected to the vertical output line 16 via the selectcircuit 130.

In the configuration example of FIG. 2, the control signal lines 14 oneach row include a reset signal line RES, a select signal line SEL, anda gain control signal line GAIN. The reset signal line RES is connectedto the counter circuits 120 of the pixels 12 belonging to the associatedrow, respectively. The select signal line SEL is connected to the selectcircuits 130 of the pixels 12 belonging to the associated row,respectively. The gain control signal line GAIN is connected to thephotoelectric conversion signal generation unit 110 of the pixels 12belonging to the associated row, respectively. The gain control signalline GAIN may be configured to be supplied with a gain control signalfrom a control circuit (not illustrated) other than the verticalscanning circuit 20. Further, the gain control signal line GAINconnected to the photoelectric conversion signal generation units 110 ofa plurality of pixels 12 belonging to the same row is not necessarilyrequired to be the common signal line.

FIG. 3 is a circuit diagram illustrating a configuration example of thephotoelectric conversion signal generation unit 110. The photoelectricconversion signal generation unit 110 includes a plurality of pixelelements each including an avalanche photodiode APD, a p-channel MOStransistor MP, an n-channel MOS transistor MN, and an inverter circuitINV and includes an OR circuit 112. FIG. 3 illustrates the photoelectricconversion signal generation unit 110 including three pixel elements ofa first pixel element, a second pixel element, and a third pixelelement. Note that the number of pixel elements included in thephotoelectric conversion signal generation unit 110 is not limited inparticular.

The first pixel element includes an avalanche photodiode APD1, ap-channel MOS transistor MP1, an n-channel MOS transistor MN1, and aninverter circuit INV1. The anode of the avalanche photodiode APD1 isconnected to a power source that supplies a voltage VL. The cathode ofthe avalanche photodiode APD1 (node N1) is connected to the drain of thep-channel MOS transistor MP1, the drain of the n-channel MOS transistorMN1, and the input terminal of the inverter circuit INV1. The source ofthe p-channel MOS transistor MP1 is connected to a power source thatsupplies a voltage VH whose potential is higher than the voltage VL. Thesource of the n-channel MOS transistor MN1 is grounded. The outputterminal of the inverter circuit INV1 is connected to one of the inputterminals of the OR circuit 112. The output terminal of the OR circuit112, which also serves as the output terminal of the photoelectricconversion signal generation unit 110, is connected to the countercircuit 120.

Similarly, the second pixel element includes an avalanche photodiodeAPD2, a p-channel MOS transistor MP2, an n-channel MOS transistor MN2,and an inverter circuit INV2. The anode of the avalanche photodiode APD2is connected to a power source that supplies a voltage VL. The cathodeof the avalanche photodiode APD2 (node N2) is connected to the drain ofthe p-channel MOS transistor MP2, the drain of the n-channel MOStransistor MN2, and the input terminal of the inverter circuit INV2. Thesource of the p-channel MOS transistor MP2 is connected to a powersource that supplies a voltage VH whose potential is higher than thevoltage VL. The source of the n-channel MOS transistor MN2 is grounded.The output terminal of the inverter circuit INV2 is connected to one ofthe input terminals of the OR circuit 112.

Similarly, the third pixel element includes an avalanche photodiodeAPD3, a p-channel MOS transistor MP3, an n-channel MOS transistor MN3,and an inverter circuit INV3. The anode of the avalanche photodiode APD3is connected to a power source that supplies a voltage VL. The cathodeof the avalanche photodiode APD3 (node N3) is connected to the drain ofthe p-channel MOS transistor MP3, the drain of the n-channel MOStransistor MN3, and the input terminal of the inverter circuit INV3. Thesource of the p-channel MOS transistor MP3 is connected to a powersource that supplies a voltage VH whose potential is higher than thevoltage VL. The source of the n-channel MOS transistor MN3 is grounded.The output terminal of the inverter circuit INV3 is connected to one ofthe input terminals of the OR circuit 112.

In the case of the configuration example illustrated in FIG. 3, the gaincontrol signal line GAIN connected to the photoelectric conversionsignal generation unit 110 includes three signal lines that supply gaincontrol signals ϕG1, ϕG2, and ϕG3 to the gates of the n-channel MOStransistors MN1, MN2, and MN3, respectively.

Each of the pixel elements, the p-channel MOS transistor MP functions asa quenching resistor of the avalanche photodiode APD. Further, then-channel MOS transistor MN functions as a gain control switch thatseparately controls the gain (an active state or an inactive state) ofthe avalanche photodiode APD. The inverter circuit INV shapes a waveformof a signal output from the avalanche photodiode APD into a pulsewaveform.

The imaging device according to the present embodiment can be formed byattaching a substrate in which the avalanche photodiode APDs areprovided and a substrate in which other components than the avalanchephotodiode APDs are provided as an example (see a fifth embodiment).Other components may include peripheral circuits such as the verticalscanning circuit 20, the column readout circuit 30, the horizontalscanning circuit 40, the control circuit 50, the output circuit 60, andthe like in addition to the components of the pixel 12 other than theavalanche photodiode APD. FIG. 4 illustrates a cross-sectional view of asubstrate in which the avalanche photodiodes APD are provided of thesetwo substrates.

The avalanche photodiode APD of each pixel 12 is formed in asemiconductor substrate 210. The semiconductor substrate 210 is ann-type silicon substrate, for example. The semiconductor substrate 210includes a first face 212 and a second face 214 opposing to the firstface 212. For example, the first face 212 is a front face of thesemiconductor substrate 210, and the second face 214 is a back face ofthe semiconductor substrate 210.

In the semiconductor substrate 210, n-type semiconductor regions 224 areprovided. The n-type semiconductor regions 224 are divided by isolationportions 220 into regions each corresponding to one of the pixels 12. Ineach of the n-type semiconductor regions 224 divided by the isolationportions 220, a plurality of avalanche photodiodes APD included in eachpixel 12 are arranged.

The isolation portion 220 includes a p-type semiconductor region 216 incontact with the first face 212 of the semiconductor substrate 210 and ap-type semiconductor region 218 arranged in contact with the p-typesemiconductor region 216 on the second face 214 side and is arranged soas to surround each n-type semiconductor region 224 in a planar view.The second face 214 side of the p-type semiconductor region 218 is incontact with the p-type semiconductor region 222 provided in contactwith the second face 214 of the semiconductor substrate 210.

Note that, while the example in which the n-type semiconductor region224 in which a plurality of avalanche photodiodes APD are arranged ineach pixel 12 is isolated by p-n junction isolation is illustrated here,other element isolation methods may be used for isolation. Other elementisolation methods may be a shallow trench isolation (STI) method, a deeptrench isolation (DTI) method, a local oxidation of silicon (LOCOS)method, or the like, for example.

In each n-type semiconductor region 224 defined by the isolationportions 220, a plurality of avalanche photodiodes APD included in onepixel 12 are arranged. FIG. 4 illustrates three avalanche photodiodesAPD arranged in the n-type semiconductor region 224.

Each of the avalanche photodiodes APD includes n-type semiconductorregions 226 and 228 and p-type semiconductor regions 230 and 232. Then-type semiconductor region 226 is arranged in contact with the firstface 212 of the semiconductor substrate 210. The n-type semiconductorregion 228 is arranged in contact with the first face 212 of thesemiconductor substrate 210 so as to surround the n-type semiconductorregion 226. The p-type semiconductor region 230 is arranged in contactwith the bottom of the n-type semiconductor region 228. The p-typesemiconductor region 232 is arranged in contact with the first face 212of the semiconductor substrate 210 between the n-type semiconductorregions of the adjacent avalanche photodiodes APD.

In this example, the impurity concentration of the n-type semiconductorregion 226 is higher than the impurity concentration of the n-typesemiconductor region 228. Further, the impurity concentration of then-type semiconductor region 228 is higher than the impurityconcentration of the n-type semiconductor region 224. The impurityconcentration of the p-type semiconductor region 216 is higher than theimpurity concentrations of the p-type semiconductor region 218 and thep-type semiconductor region 222. Further, the impurity concentrations ofthe p-type semiconductor region 218 and the p-type semiconductor region222 are higher than the impurity concentrations of the p-typesemiconductor regions 230 and 232.

A sufficient higher impurity concentration of the n-type semiconductorregion 226 allows for a higher electric field of a depletion layeroccurring in the n-type semiconductor region 226. The n-typesemiconductor region 228 having a lower concentration than the n-typesemiconductor region 226 has a function of an electric field mitigationregion for not causing the electric filed concentration region of then-type semiconductor region 226 to expand to other regions. The p-typesemiconductor regions 230 and 232 form a potential valley for guidingcharges (electrons) occurring inside the n-type semiconductor region 224to the n-type semiconductor region 224. The n-type semiconductor region224 is a photoelectric conversion region that generates carriers from aphoton incidence.

On the semiconductor substrate 210 in which the avalanche photodiodesAPD are provided, an insulating film 234 is provided. On the insulatingfilm 234, interconnections 238 connected to the p-type semiconductorregions 216 are provided via contact plugs 236, and interconnections 242connected to the n-type semiconductor regions 226 are provided viacontact plugs 240. The voltage VL is supplied to the p-typesemiconductor regions 216 via the interconnections 238 and the contactplugs 236. The n-type semiconductor regions 226 are connected to thenodes N of the circuit illustrated in FIG. 3 via the contact plugs 240and the interconnections 242.

Next, a method of driving the imaging device according to the presentembodiment will be described with an example of the case of the circuitconfiguration of FIG. 3.

The voltage VH and the voltage VL are set to be able to apply a reversebias voltage that is sufficient for the avalanche photodiode APD tooperate in the Geiger mode. As an example here, it is assumed that theavalanche photodiode APD has a current-to-voltage characteristic asillustrated in FIG. 5. The reverse breakdown voltage of the avalanchephotodiode APD illustrated in FIG. 5 occurs between −50 V and −53.3 V.That is, while avalanche amplification does not occur in this avalanchephotodiode APD when a reverse bias voltage of around −50 V is applied,avalanche amplification occurs when a reverse bias voltage greater thanaround −50 V is applied. Further, when a reverse bias voltage greaterthan around −53.3 V is applied, this causes an operation in a so-calledGeiger mode in which the gain of avalanche amplification becomessignificantly higher.

In the present embodiment, each avalanche photodiode APD operates in theGeiger mode, that is, is used as a single-photon avalanche diode (SPAD).Thus, for example, −50 V is applied as the voltage VL to the anode ofthe avalanche photodiode APD, and, for example, +3.3 V is applied as thevoltage VH to the source of the p-channel MOS transistor MP.

Typically, the same voltage as the voltage applied to the source isapplied to the gate of the p-channel MOS transistor MP serving as aquenching resistor. This causes the p-channel MOS transistor MP tooperate as a resistance element whose resistance is determined by thedimension of the transistor. It is possible to intentionally set thevoltage applied to the gate of the p-channel MOS transistor MP lowerthan the voltage applied to the source to utilize the p-channel MOStransistor MP as a resistance element having a higher resistance. Thegate voltage of the p-channel MOS transistor MP can be set asappropriate so that a desired quenching resistance can be obtained.

The n-channel MOS transistor MN as a gain control switch is controlledby the gain control signal ϕG supplied to the gate thereof. Then-channel MOS transistor MN is in an on-state when the gain controlsignal ϕG is at a high level, and is in an off-state when gain controlsignal ϕG is at a low level.

When the n-channel MOS transistor MN is in an off-state, in an initialstate where no current flows, a reverse bias voltage of 53.3 V, which isthe potential difference between the voltage VH and the voltage VL, isapplied to the avalanche photodiode APD. Although this reverse biasvoltage is higher than the breakdown voltage and is sufficient to causeavalanche amplification, no avalanche amplification occurs in a statewhere no carrier to be a seed is present, and thus no current flows inthe avalanche photodiode APD.

Once photons enter the second face 214 side of the semiconductorsubstrate 210 in this state, the photons are absorbed in the n-typesemiconductor region 224, and electron and hole pairs are generated.Holes of these pairs are drained via the p-type semiconductor regions216, 218, 222, 230, and 232. On the other hand, electrons are guided tothe n-type semiconductor region 226 along the potential valley betweenthe p-type semiconductor regions 230. These electrons are accelerated bythe electric field in a part of the n-type semiconductor region 226 tocause avalanche amplification, and the avalanche photodiode APD operatesin the Geiger mode.

A large current flows in the avalanche photodiode APD ensuing avalancheamplification, and thereby the potential of the node N (nodes N1, N2,N3) that is the terminal on the cathode side of the avalanche photodiodeAPD decreases and the avalanche amplification stops. Carriers at thenode N are gradually drained via the p-channel MOS transistor MPconnected as a load, and the voltage of the node N returns to theinitial voltage (quenching operation).

In such a way, in response to a photon incidence, the potential of thecathode (node N) of the avalanche photodiode APD changes from a carrierwaiting state to a state where the voltage has decreased due to a flowof a large current in a Geiger mode and then returns to the carrierwaiting state. By shaping this voltage waveform at the node N by theinverter circuit INV, a signal pulse starting from the arrival time ofone photon is generated. By counting the number of these signal pulses,so-called photon-counting can be performed.

On the other hand, when the n-channel MOS transistor MN is in anon-state, the potential of the cathode (node N) of the avalanchephotodiode APD is 0 V, and the reverse bias voltage applied to theavalanche photodiode APD is 50 V that is a voltage below the breakdownvoltage. In this state, even when electrons occur due to a photonincidence, these electrons do not cause avalanche amplification and aredrained via the p-channel MOS transistor MP. Since a current due toelementary charges is sufficiently small and a voltage drop at thep-channel MOS transistor MP is sufficiently small, the threshold valueof the inverter circuit INV is not exceeded and no signal pulse isoutput.

In such a way, the n-channel MOS transistor MN is a switch that switchesthe voltage applied to the avalanche photodiode APD to a voltage that ishigher than the breakdown voltage or less than or equal to the breakdownvoltage of the avalanche photodiode APD.

The avalanche photodiodes APD1, APD2, and APD3 are able to detectphotons independently of each other, and signal pulses are output fromthe inverter circuits INV1, INV2, and INV3, respectively. The outputs ofthe inverter circuits INV1, INV2, and INV3 are bundled into one by theOR circuit 112 and output to the counter circuit 120. Thereby, in thecounter circuit 120, the total number of photons determined by theavalanche photodiodes APD1, APD2, and APD3 is counted. The count valuecounted by the counter circuit 120 is then output to the vertical outputline 16 with the select circuit 130 operating in accordance with theselect signal from the vertical scanning circuit 20.

Next, a photon-counting operation in the imaging device according to thepresent embodiment will be described more specifically by using FIG. 6to FIG. 7B. Note that, in the operation described below, it is assumedthat the gain control signals ϕG of the n-channel MOS transistors MN1,MN2, and MN3 as gain control switches are at a low level, and theavalanche photodiodes APD1, APD2, and APD3 operate in the Geiger mode.

FIG. 6 is a graph illustrating the relationship between the number ofincident photons and the photon count by the counter circuit 120. Asillustrated in FIG. 6, when the number of incident photons is small, thenumber can be accurately counted by the counter circuit 120 (region A).As the number of incident photons increases, however, the photon countby the counter circuit 120 becomes smaller than the number of actualincident photons. When a light intensity is extremely high and thus thenumber of incident photons is significantly large, photon-counting bythe counter circuit 120 cannot be made, and thus the photon countbecomes zero (region B).

The reason for occurrence of the phenomenon as illustrated in FIG. 6will be described by using FIG. 7A and FIG. 7B. FIG. 7A and FIG. 7B aregraphs illustrating temporal changes of output voltages of the invertercircuits INV1, INV2, and INV3 and the OR circuit 112. FIG. 7Aillustrates a case corresponding to the region A of FIG. 6, and FIG. 7Billustrates a case corresponding to the region B of FIG. 6.

In the case of the region A, since the number of incident photons isrelatively small, there is a sufficiently smaller probability that thetimings of photons being detected by the avalanche photodiodes APD1,APD2, and APD3 temporally overlap with each other as illustrated in FIG.7A. Thus, the signal pulses output from the inverter circuits INV1,INV2, and INV3 do not temporally overlap with each other, and the ORcircuit 112 can output signal pulses the number of which corresponds tothe number of photons determined by the avalanche photodiodes APD1,APD2, and APD3. For example, in the example of FIG. 7A, while the numberof photons determined by the avalanche photodiodes APD1, APD2, and APD3(the number of output pulses of the inverter circuits INV1, INV2, andINV2) is five, the number of signal pluses output from the OR circuit112 is also five.

On the other hand, in the case of the region B, since the number ofincident photons is large, the timings of photons being detected by theavalanche photodiodes APD1, APD2, and APD3 may temporally overlap witheach other. Further, a next incident photon occurs before a signal pulseof the OR circuit 112 completely falls, and thereby signal pulses whichhave to be detected as a plurality of signal pulses may overlap witheach other and be detected as a single continuous signal pulse. Forexample, in the example of FIG. 7B, while the number of photonsdetermined by the avalanche photodiodes APD1, APD2, and APD3 (the numberof output pulses of the inverter circuits INV1, INV2, and INV2) is 12,the number of signal pluses output from the OR circuit 112 decreases tofive.

With a further intense light, in each of the inverter circuits INV, aphoton is again detected after a signal pulse rises and before thesignal pulse falls, and thereby the next signal pulse may be continuousthereto. When such a phenomenon occurs in each of the inverter circuitsINV, this results in a state where any of the outputs of the invertercircuits INV1, INV2, and INV3 is at a high level all the time, and thusthe output of the OR circuit 112 does not change from the high level. Asa result, signal pulses cannot be counted in the counter circuit 120,which causes the photon count to be zero.

In such a way, in the photon-counting, the arrival frequency ofdetectable photons (equal to the light intensity) is determined by aperiod from the time when a photon is detected to the time when a nextphoton can be detected (called “dead time”).

In order to realize an ultrasensitive imaging device that can detect aphoton even in a significantly dark situation such as under the moonlight or star lights, it is conceivable to provide the pixels 12 thatare large to some extent. With the imaging device configured as such,however, the photon detection frequency becomes too high in a brightsituation such as under day light even when the light amount is reducedby an aperture mechanism, and thus the photon-counting cannot be made.

In terms of the above, in the imaging device according to the presentembodiment, a gain control switch is provided in each pixel element ofthe pixel 12, and the operation states of the plurality of avalanchephotodiodes APD included in a pixel can be set separately. The gaincontrol switches (n-channel MOS transistors MN) are a setting unitadapted to set a plurality of avalanche photodiodes to the active stateor the inactive state in a separate manner.

For example, in the circuit of FIG. 3, the gain control signal ϕG2 isset to a low level, and the gain control signals ϕG1 and ϕG3 are set toa high level. This allows the avalanche photodiode APD2 to be set to avoltage condition which causes avalanche amplification and the avalanchephotodiodes APD1 and APD3 to be set to a voltage condition which doesnot cause avalanche amplification. That is, the avalanche photodiodeAPD2 enters a state for responding to a light (active state), and theavalanche photodiodes APD1 and APD3 enter a state for not responding toa light (inactive state). Thereby, signals based on carriers guided tothe n-type semiconductor regions 226 of the avalanche photodiodes APD1and APD3 do not contribute to photon-counting.

For example, in the example of FIG. 7B, both the numbers of signalpulses output from the inverter circuits INV1 and INV3 are zero. As aresult, the number of signal pulses output from the OR circuit 112 isequal to four that is the number of signal pulses output from theinverter circuit INV2, which allows for accurate photon-counting.

Causing some of the plurality of avalanche photodiodes APD to enter theinactive state is equal to switching the pixel to be a small-sized pixelhaving a low sensitivity. That is, in a very dark situation with a faintlight, all the avalanche photodiodes APD are controlled to operate inthe Geiger mode so that incident photons can be detected without lossallowing for ultrasensitive capturing. On the other hand, in a brightsituation with an intense light such as daytime, some avalanchephotodiodes APD are controlled to the inactive state to performcapturing with low sensitivity. In this case, light amount control usingan aperture may be combined. By doing so, the light amount rangeenabling capturing can be expanded.

In terms of improvement of the in-plane uniformity of the sensitivity,it is desirable that the number of avalanche photodiodes APD set in theactive state be the same over all the pixels 12. The scheme for havingthe same number of avalanche photodiodes APD set in the active state inall the pixels 12 is not limited in particular. For example, when theconfiguration example illustrated in FIG. 3, there may be a scheme inwhich three signal lines that supply the gain control signals ϕG1, ϕG2,and ϕG3 to the n-channel MOS transistors MN1, MN2, and MN3 are arrangedas common signal lines to respective pixels 12. The signal lines thatsupply the gain control signals ϕG1, ϕG2, and ϕG3 may be separate signallines for respective pixels 12, and the gain control signals ϕG1, ϕG2,and ϕG3 may be set so that the total number of the avalanche photodiodesAPD set to the active state is the same over the entire pixels 12.

Note that, while the example in which the number of avalanchephotodiodes APD included in one pixel 12 is three has been described inthe present embodiment, the number of avalanche photodiodes APD includedin one pixel 12 is not limited in particular as long as it is plural.Further, the relationship between the number of avalanche photodiodesAPD in the active state and the number of avalanche photodiodes APD inthe inactive state can be selected as appropriate in accordance withcapturing conditions.

The arrangement of the avalanche photodiodes APD set in the active statemay be different for each pixel 12. FIG. 8 is a plan view of thesemiconductor substrate 210 viewed from the first face 212 side, whichillustrates a state where the n-type semiconductor regions 224 of fourpixels 12 are arranged adjacent via the isolation portions 220. Fiveavalanche photodiodes APD are arranged in each of the n-typesemiconductor regions 224 of respective pixels 12. In FIG. 8, hatchedcircles represent the avalanche photodiodes APD in the active state, andwhite circles represent the avalanche photodiodes APD in the inactivestate. In any of the pixels 12, three of the five avalanche photodiodesAPD are in the active state, and two of the five avalanche photodiodesAPD are in the inactive state. However, the positions of the avalanchephotodiodes APD in the active state are different for respective pixels12. While it is desirable that the number of avalanche photodiodes APDset in the active state be the same in obtaining the same sensitivity ofthe pixels 12, the positions of the avalanche photodiodes APD set in theactive state are not limited in particular.

When the imaging device 100 is mounted in an imaging system such as adigital single lens reflex camera, a control signal in accordance withthe brightness of an object is supplied to the imaging device 100 fromthe imaging system side. In this case, for example, the control circuit50 functions as a control signal receiving unit that receives thecontrol signal and controls the number of the avalanche photodiodes APDset in the active state in accordance with the control signal.

As discussed above, according to the present embodiment, since eachpixel is formed of a plurality of avalanche photodiodes and theplurality of avalanche photodiodes are separately set to the activestate or the inactive state, the flexibility of setting of thephotoelectric conversion gain can be improved. Further, with the numberof avalanche photodiodes in the active state being the same in aplurality of pixels of a pixel array, a uniform sensitivity of thepixels can be obtained, and the image quality can be improved.

Second Embodiment

An imaging device and a method of driving the same according to a secondembodiment of the present invention will be described with reference toFIG. 9. Similar components to those of the imaging device according tothe first embodiment are labeled with the same reference symbols, andthe description thereof will be omitted or simplified. FIG. 9 is acircuit diagram illustrating a configuration example of a photoelectricconversion signal generation unit of a pixel of the imaging deviceaccording to the present embodiment.

The imaging device according to the present embodiment is similar to theimaging device according to the first embodiment except a difference inthe configuration of the photoelectric conversion signal generation unit110. That is, as illustrated in FIG. 9, the photoelectric conversionsignal generation unit 110 of the imaging device according to thepresent embodiment does not have the n-channel MOS transistors MN1, MN2,and MN3 as the gain control switch. Instead, switches SW1, SW2, and SW3are provided between the inverter circuits INV1, INV2, and INV3 and theOR circuit 112. The switches SW1, SW2, and SW3 can be controlled by thegain control signals ϕG1, ϕG2, and ϕG3, respectively. For example, whenthe gain control signal ϕG is at a high level, the corresponding switchSW is in an on-state, and when the gain control signal ϕG is at a lowlevel, the corresponding switch SW is in an off-state.

In the present embodiment, a reverse bias voltage of 53.3 V that is thepotential difference between the voltage VH and the voltage VL is alwaysapplied to the avalanche photodiodes APD, and all the avalanchephotodiodes APD always operate in the Geiger mode. Thus, when the numberof incident photons is large, the signal pulses output from the invertercircuits INV1, INV2, and INV3 are as illustrated in FIG. 7B, forexample.

However, the switches SW1, SW2, and SW3 are provided between theinverter circuits INV1, INV2, and INV3 and the OR circuit 112, and anyof the inverter circuits INV can be selected to be connected to the ORcircuit 112. For example, when the switch SW2 is set to an on-state andthe switches SW1 and SW3 are set to an off-state, only the signal pulseoutput from the inverter circuit INV2 is input to the OR circuit 112.Thereby, the number of signal pulses output from the OR circuit 112 willbe equal to four that is the number of signal pulses output from theinverter circuit INV2, which allows for accurate photon-counting.Thereby, the same advantages as obtained in the first embodiment can berealized.

The switches SW are not necessarily required to be provided between theinverter circuits INV and the OR circuit 112, and may be providedbetween the cathode terminals (nodes N) of the avalanche photodiodes APDand the inverter circuits INV. In this case, any of the avalanchephotodiodes APD can be selected to be connected to the OR circuit 112via the inverter circuits INV, the same effects and advantages asobtained in the circuit of FIG. 9 can be realized.

In such a way, the switches SW are switches that switch connection anddisconnection between the avalanche photodiodes APD and the OR circuit112. In other words, the switches SW form a setting unit adapted toseparately set a plurality of avalanche photodiodes to the active stateor the inactive state. Note that the OR circuit 112 can be considered asa part of the counter circuit 120. In this case, it can be said that theswitches SW are switches that switch connection and disconnectionbetween the avalanche photodiodes APD and the counter circuit 120.

Note that, when the connection between the avalanche photodiode APD andthe OR circuit 112 is opened by the switch SW, the output side terminalof the switch SW is in a floating state. For example, in the circuitconfiguration of FIG. 9, of the OR circuit 112, an input terminalconnected to the switch SW which is in an off-state of the inputterminals is in a floating state. When the potential of this inputterminal fluctuates due to a disturbance or the like, this may affectthe output signal of the OR circuit 112 resulting in inaccuratephoton-counting.

Therefore, in the case of the configuration in which the switches SW areprovided between the avalanche photodiodes APD and the OR circuit 112 asdescribed in the present embodiment, it is desirable that the terminalon output side of the switch SW which is in an off-state be fixed to apredetermined potential. For example, in the case of the circuitconfiguration of FIG. 9, the potential of the input terminal of the ORcircuit 112 connected to the switch SW which is in an off-state is fixedto a low level.

As discussed above, according to the present embodiment, since eachpixel is formed of a plurality of avalanche photodiodes and theplurality of avalanche photodiodes are separately set in the activestate or the inactive state, the flexibility of setting of thephotoelectric conversion gain can be improved. Further, with the numberof avalanche photodiodes in the active state being the same in aplurality of pixels of a pixel array, a uniform sensitivity of thepixels can be obtained, and the image quality can be improved.

Third Embodiment

A method of driving an imaging device according to a third embodiment ofthe present invention will be described with reference to FIG. 10A toFIG. 10C. Similar components to those of the imaging device according tothe first and second embodiments are labeled with the same referencesymbols, and the description thereof will be omitted or simplified. FIG.10A to FIG. 10C are diagrams illustrating the method of driving theimaging device according to the present embodiment.

In the present embodiment, another method of driving the imaging deviceaccording to the first or second embodiment will be described. Theconfiguration of the imaging device is the same as the imaging device ofthe first or second embodiment.

In the method of driving the imaging device according to the presentembodiment, the arrangement of the avalanche photodiodes APD set to theactive state and the avalanche photodiodes APD set to the inactive statewithin each one pixel is changed with time.

FIG. 10A schematically illustrates a view in which a whole exposureperiod for acquiring one image is divided into 2N periods (N is aninteger), and the divided exposure period 1, exposure period 2, exposureperiod 3, . . . , and exposure period 2N are sequentially performedaccording to elapsing of time t. The references “pattern 1” and “pattern2” depicted under each exposure period represent arrangement pattern ofthe avalanche photodiodes APD in the active state and the avalanchephotodiodes APD in the inactive state in the corresponding exposureperiod. That is, the pattern 1 is employed in the exposure period 1, andthe pattern 2 is employed in the exposure period 2. The pattern 1 andthe pattern 2 are employed in an alternating manner in the subsequentexposure period.

FIG. 10B and FIG. 10C illustrate specific examples of the pattern 1 andthe pattern 2. FIG. 10B and FIG. 10C are plan views of the semiconductorsubstrate 210 viewed from the first face 212 side, which illustrate fiveavalanche photodiodes APD of one pixel 12 that are arranged inside then-type semiconductor region 224. In FIG. 10B and FIG. 10C, hatchedcircles represent the avalanche photodiodes APD in the active state, andwhite circles represent the avalanche photodiodes APD in the inactivestate. Each circle depicted with a dotted line about each avalanchephotodiode APD is schematic illustration of a range of the photon thatcan be collected by the avalanche photodiode APD present at the center.In the actual implementation, the intermediate part between theavalanche photodiodes APD is an approximate ridge of the potential, andphotons within the region surrounded by the ridges converge at eachavalanche photodiode APD.

In the pattern 1, two right-side avalanche photodiodes APD in additionto the center avalanche photodiode APD are set to the active state, andthereby the pattern 1 provides an operation to facilitate detection ofphotons incident at the right-side region. On the other hand, in thepattern 2, two left-side avalanche photodiodes APD in addition to thecenter avalanche photodiode APD are set to the active state, and therebythe pattern 2 provide an operation to facilitate detection of photonsincident at the left-side region.

When a part of the plurality of avalanche photodiodes APD included inone pixel 12 is set to the active state, a state may occur where photonsare detected in a particular region within a pixel region. In such acase, with arrival positions of a light to the pixels 12 beingunbalance, the number of photons incident at the pixel 12 may not beaccurately counted. For example, in an area-type sensor, the lightincident angle may be different between the left side and the right sidein the pixels 12 in the left-side and the right-side circumferentialportions of the screen, which may cause unbalanced arrival positions ofa light to the pixels 12.

The whole exposure period is divided into a plurality of exposureperiods and the pattern of the avalanche photodiodes APD to be set tothe active state/the inactive state for respective exposure periods ischanged, and thereby an average signal output can be obtained even withthe unbalance arrival position of a light to the pixels 12.

The number or the arrangement of the avalanche photodiodes APD in theactive state in respective divided exposure periods is not limited tothe example described above. For example, the number of avalanchephotodiodes APD included in each one pixel 12 is not limited to five.Further, while the example in which the detection area is switched in alateral direction for respective exposure periods has been described inthe present embodiment, the detection area may be switched in a verticaldirection or in a diagonal direction. Further, the plurality ofavalanche photodiodes APD included in each one pixel 12 may be operatedseparately one by one. Further, while two types of patterns are repeatedin the present embodiment, three or more types of patterns may berepeated.

As discussed above, according to the present embodiment, since thearrangement of the avalanche photodiodes set in the active state withineach one pixel during an exposure period for acquiring one image isswitched, it is possible to suppress unbalanced arrival positions of alight to the pixels from affecting the image quality.

Fourth Embodiment

A method of driving an imaging device according to a fourth embodimentof the present invention will be described with reference to FIG. 11A toFIG. 11D. Similar components to those of the imaging device according tothe first to third embodiments are labeled with the same referencesymbols, and the description thereof will be omitted or simplified. FIG.11A to FIG. 11D are diagrams illustrating the method of driving theimaging device according to the present embodiment.

In the present embodiment, another method of driving the imaging deviceaccording to the first or second embodiment will be described. Theconfiguration of the imaging device is the same as the imaging device ofthe first or second embodiment.

In the method of driving the imaging device according to the presentembodiment, the arrangement of the avalanche photodiodes APD set to theactive state and the avalanche photodiodes APD set to the inactive statewithin each one pixel is changed with time likewise the case of thethird embodiment. In the present embodiment, however, the number ofavalanche photodiodes APD set to the active state is changed in thedivided exposure period, if necessary.

FIG. 11A schematically illustrates a view in which a whole exposureperiod for acquiring one image is divided into 3N periods (N is aninteger), and the divided exposure period 1, exposure period 2, exposureperiod 3, . . . , and exposure period 3N are sequentially performedaccording to elapsing of time t. The references “pattern 1”, “pattern2”, and “pattern 3” depicted under each exposure period representarrangement pattern of the avalanche photodiodes APD in the active stateand the avalanche photodiodes APD in the inactive state in thecorresponding exposure period. That is, the pattern 1 is employed in theexposure period 1, the pattern 2 is employed in the exposure period 2,and the pattern 3 is employed in the exposure period 3. In thesubsequent exposure periods, the pattern 1, the pattern 2, and thepattern 3 are repeatedly employed in this order.

FIG. 11B, FIG. 11C, and FIG. 11D illustrate specific examples of thepattern 1, the pattern 2, and the pattern 3. FIG. 11B, FIG. 11C, andFIG. 11D are plan views of the semiconductor substrate 210 viewed fromthe first face 212 side, which illustrate five avalanche photodiodes APDof one pixel that are arranged inside the n-type semiconductor region224. In FIG. 11B, FIG. 11C, and FIG. 11D, hatched circles represent theavalanche photodiodes APD in the active state, and white circlesrepresent the avalanche photodiodes APD in the inactive state. Eachcircle depicted with a dotted line about each avalanche photodiode APDis schematic illustration of a range of the photon that can be collectedby the avalanche photodiode APD present at the center.

In the pattern 1, two right-side avalanche photodiodes APD in additionto the center avalanche photodiode APD, namely, three avalanchephotodiodes APD in total are set to the active state. In the pattern 2,two left-side avalanche photodiodes APD in addition to the centeravalanche photodiode APD, namely, three avalanche photodiodes APD intotal are set to the active state. In the pattern 3, four avalanchephotodiodes APD except the center avalanche photodiode APD, namely, fouravalanche photodiodes APD in total are set to the active state. In otherwords, in the pattern 1 and the pattern 2, ⅗ of the five avalanchephotodiodes APD are set to the active state. In the pattern 3, ⅘ of thefive avalanche photodiodes APD are set to the active state.

The whole exposure period is divided into a plurality of exposureperiods while the number of avalanche photodiodes APD in the activestate for exposure is changed in such a way, which can improve theflexibility of setting of the sensitivity. For example, when thesensitivity with all the avalanche photodiodes APD being in active stateis 1, it is possible to set the sensitivity to ½ or ⅓ thereof.

For example, when all the five avalanche photodiodes APD are set to theactive state from the exposure period 1 to the exposure period 3, thetotal number of 15 avalanche photodiodes APD are set to the active statefrom the exposure period 1 to the exposure period 3. On the other hand,when the pattern 1 to the pattern 3 illustrated in FIG. 11B to FIG. 11Dare employed as the patterns from the exposure period 1 to the exposureperiod 3, the total number of 10 avalanche photodiodes APD are set tothe active state from the exposure period 1 to the exposure period 3.

Therefore, by setting the whole exposure period as illustrated in FIG.11A, the sensitivity can be reduced to ⅔ compared to the case where allthe five avalanche photodiodes APD are set to the active state. Such anoperation allows for sensitivity setting such as “−⅓ steps”, forexample, similarly to the ISO sensitivity setting in a camera.

As discussed above, according to the present embodiment, since thenumber and the arrangement of the avalanche photodiodes set in theactive state within each one pixel during an exposure period foracquiring one image are switched, the flexibility of the sensitivitysetting of the pixel can be improved. This allows the sensitivity of thepixel to be set suitably in accordance with the ISO sensitivity settingof a camera.

Fifth Embodiment

An imaging device according to a fifth embodiment of the presentinvention will be described with reference to FIG. 12. Similarcomponents to those of the imaging device according to the first tofourth embodiments are labeled with the same reference symbols, and thedescription thereof will be omitted or simplified. FIG. 12 is across-sectional view illustrating the general configuration of theimaging device according to the present embodiment.

As previously described, the imaging device 100 of the present inventioncan be formed by attaching a substrate in which the avalanchephotodiodes APD are provided and a substrate in which other componentsthan the avalanche photodiodes APD are provided, for example. In thepresent embodiment, an example of the imaging device 100 formedincluding the substrate in which the avalanche photodiodes APD areprovided and the substrate in which other components than the avalanchephotodiodes APD are provided will be illustrated.

As illustrated in FIG. 12, the imaging device 100 includes the substrate200 in which the avalanche photodiodes APD are provided and thesubstrate 300 in which other components than the avalanche photodiodesAPD are provided. The substrate 200 is a substrate in which a pluralityof interconnection layers are further formed if necessary on theinsulating film 234 arranged over the semiconductor substrate 210.Interconnections 250 in the uppermost layer provided in the substrate200 are electrically connected to the interconnections 242 via aplurality of interconnections or contact plugs arranged inside theinsulating film 252. The substrate 300 is a substrate in which aplurality of interconnection layers are formed on the semiconductorsubstrate 310 in which a control unit 312 including other componentsthan the avalanche photodiodes APD is provided. The interconnections 320in the uppermost layer provided in the substrate 300 are electricallyconnected to the control unit 312 via a plurality of interconnections orcontact plugs arranged inside the insulating film 322. The substrate 200and the substrate 300 are attached such that the interconnections 250 ofthe substrate 200 and the interconnections 320 of the substrate 300 areconnected to each other.

A color filter 330 and microlenses 340 are arranged on the second face214 side of the semiconductor substrate 210. The embodiment isconfigured such that the light entering the imaging device 100 passesthrough the microlenses 340 and the color filter 330 and enters then-type semiconductor region 224 from the second face 214 side of thesemiconductor substrate 210. That is, in the present embodiment, theavalanche photodiode APD is configured as the backside illuminationtype.

In the present embodiment, since the components other than the avalanchephotodiodes APD are arranged in the substrate 300 separate from thesubstrate 200 in which the avalanche photodiodes APD are provided, theflexibility of the arrangement of the avalanche photodiodes APD isimproved. Thereby, the aperture ratio of the avalanche photodiodes APDcan be easily increased, and thus the light detection efficiency can beimproved.

Note that, while the avalanche photodiodes APD are configured asbackside illumination type in the present embodiment, configuration of afront side illumination type may be employed. However, the backsideillumination type is preferred to the front side illumination type,because charges generated near the surface of the substrate (on thelight incidence side) can be detected at a higher efficiency andtherefore a higher light detection efficiency can be realized in a broadwavelength band from a short wavelength to a long wavelength in thebackside illumination type than in the front side illumination type.

Sixth Embodiment

A method of driving an imaging device according to a sixth embodiment ofthe present invention will be described with reference to FIG. 13 andFIG. 14. Similar components to those of the imaging device according tothe first to fifth embodiments are labeled with the same referencesymbols, and the description thereof will be omitted or simplified. FIG.13 and FIG. 14 are flowcharts illustrating the method of driving theimaging device according to the present embodiment.

The imaging device described in the first to fifth embodiments can beapplied to various imaging systems as illustrated in the embodimentsdescribed later. In the present embodiment, an example of driving theimaging device when any of the imaging devices described in the first tofifth embodiments is applied to an imaging system will be described.

FIG. 13 is a control flow of the imaging device in an imaging systemhaving a function of capturing a static image as represented by adigital single-lens reflex camera.

A digital single-lens reflex camera mounts an AE sensor or the like inaddition to the imaging device therein and has an auto setting mode thatreferences a setting table or the like included in the camera inaccordance with the amount of a collected light and determines anaperture value of a lens, exposure time, or an ISO sensitivity setting.Further, a digital single-lens reflex camera has a manual setting modethat determines photographing conditions in accordance with settings bythe user.

In the auto setting mode, first, in step S101, a light receiving elementsuch as an AE sensor is used to determine the brightness of a scene.

Next, in step S102, an exposure level is determined based on thebrightness of the scene determined in step S101.

Next, in step S103, a setting table in the camera is referenced to setsetting values of the aperture value of the lens (Av value), the shutterspeed (Tv value), and the ISO sensitivity to the optimal values inaccordance with the exposure level determined in step S102.

Next, in step S104, out of a plurality of avalanche photodiodes APDincluded in each one pixel, the number of avalanche photodiodes APD tobe set to the active state is set in accordance with the ISO sensitivityset in step S103.

Next, in step S105, the drive pattern in the exposure period is set inaccordance with the shutter speed (Tv value) set in step S103 and thenumber of avalanche photodiodes APD to be set to the active state set instep S104.

Next, in step S106, actual capturing is performed.

On the other hand, in the manual setting mode, in step S107, settingentry of any ISO sensitivity is accepted through a button operation orthe like by the user. Then, actual capturing is performed in a similarmanner to step S104 to step S106 described above.

FIG. 14 is a control flow in an imaging system having a function ofcontinuously acquiring an image while performing sensitivity controlfollowing a change in the brightness of a scene, such as when capturinga motion image by a video camera, for example.

In light collection using an AE sensor as applied in capturing a staticimage, since a light path of an incident light on a camera needs to bechanged and guided to a light receiving element, light collection cannotbe made when the signals are required to be continuously output such aswhen a motion image is captured. Thus, in capturing a motion image, thesignal level of a captured frame image is determined to change a settingon and after the next frame if necessary.

In step S201, upon the start of capturing, the first frame image isacquired in the subsequent step S202.

Next, in step S203, an average signal output x is calculated that is anaveraged signal level of the pixels in a designated region of theacquired frame image, for example, a region of 100 pixels by 100 pixelsat the center portion.

Next, in step S204, threshold value determination of the average signaloutput x is performed. Specifically, it is determined whether or not theaverage signal output x is within a range between the lower limit valueL2 and the upper limit value L1 of a predefined signal level.

As a result, if the average signal output x is within the range betweenthe lower limit value L2 and the upper limit value L1 (L2≤x≤L1), it isdetermined that the exposure is appropriate and no change is necessaryin the exposure setting, and the process returns to step S202 to acquirea frame image of the next frame.

On the other hand, overexposure is determined if the average signaloutput x is greater than the upper limit value L1 (L1<x), whileunderexposure is determined if the average signal output x is less thanthe lower limit value L2 (x<L2), and step S205 is entered.

In step S205, based on the determination result of the exposure in stepS204, setting values of the aperture value of the lens (Av value), ashutter speed (Tv value), and an ISO sensitivity are set to the optimalvalue.

Next, in step S206, out of a plurality of avalanche photodiodes APDincluded in each one pixel, the number of avalanche photodiodes APD tobe set to the active state is set in accordance with the ISO sensitivityset in step S205.

Next, in step S207, the drive pattern in the exposure period is set inaccordance with the shutter speed (Tv value) set in step S205 and thenumber of avalanche photodiodes APD to be set to the active state set instep S207. Then, the process returns to step S202 to acquire a frameimage of the next frame.

Note that, while the exposure is controlled for each frame of a motionimage according to the above flow, it is possible to perform such anoperation that the exposure setting is re-set once for capturing of 10frames, for example, in the actual implementation.

Seventh Embodiment

An imaging system according to a seventh embodiment of the presentinvention will be described with reference to FIG. 15. Similarcomponents to those of the imaging device according to the first tosixth embodiments are labeled with the same reference symbols, and thedescription thereof will be omitted or simplified. FIG. 15 is a blockdiagram illustrating a general configuration of the imaging systemaccording to the present embodiment.

The imaging devices 100 described in the above first to sixthembodiments can be applied to various imaging systems. Examples of theapplicable imaging systems may include a digital still camera, a digitalcamcorder, a surveillance camera, a copy machine, a fax machine, amobile phone, an on-vehicle camera, an observation satellite, and thelike. Further, a camera module having an optical system, such as a lens,and an imaging device may be included in the imaging system. FIG. 15illustrates a block diagram of a digital still camera as an example ofthe above.

The imaging system 400 illustrated as an example in FIG. 15 includes animaging device 402, a lens 404 that captures an optical image of anobject onto the imaging device 402, an aperture 406 for changing a lightamount passing through the lens 404, and a barrier 408 for protectingthe lens 404. The lens 404 and the aperture 406 form an optical systemthat converges a light onto the imaging device 402. The imaging device402 is the imaging device 100 described in the first to sixthembodiments and converts an optical image captured by the lens 404 intoimage data.

The imaging system 400 further includes a signal processing unit 410that processes an output signal output from the imaging device 402. Thesignal processing unit 410 performs AD conversion that converts ananalog signal output from the imaging device 402 into a digital signal.Further, the signal processing unit 410 performs other operations ofperforming various correction or compression if necessary and outputtingimage data. An AD conversion unit that is a part of the signalprocessing unit 410 may be formed on the semiconductor substrate (forexample, semiconductor substrate 310) in which the imaging device 402 isprovided, or may be formed on a different semiconductor substrate fromthe imaging device 402. Further, the imaging device 402 and the signalprocessing unit 410 may be formed on the same semiconductor substrate.

The imaging system 400 further includes a memory unit 420 fortemporarily storing image data therein and an external interface unit(external I/F unit) 414 for communicating with an external computer orthe like. The imaging system 400 further includes a storage medium 416such as a semiconductor memory for performing storage or readout ofimage pickup data and a storage medium control interface unit (storagemedium control I/F unit) 418 for performing storage or readout on thestorage medium 416. Note that the storage medium 416 may be embedded inthe imaging system 400 or may be removable.

The imaging system 400 further includes a general control/operation unit420, a timing generation unit 422, and a gain control unit 424. Thegeneral control/operation unit 420 performs various computation andcontrols the entire digital still camera. The timing generation unit 422outputs various timing signals to the imaging device 402 and the signalprocessing unit 410. The gain control unit 424 controls the gain settingof the imaging device 402 and serves to convey the gain settinginformation to the signal processing unit 410 and the timing generationunit 422. Here, the timing signal or the like may be input from theoutside, and the imaging system 400 may have at least the imaging device402 and the signal processing unit 410 that processes an output signaloutput from the imaging device 402.

The imaging device 402 outputs an imaging signal to the signalprocessing unit 410. The signal processing unit 410 performspredetermined signal processing on an imaging signal output from theimaging device 402 and outputs image data. Further, the signalprocessing unit 410 uses an imaging signal to generate an image.

Application of the imaging device 100 of any of the first to sixthembodiments can realize an imaging system that can acquire a goodquality image with a wide range of the brightness.

Eighth Embodiment

An imaging system and a movable object according to an eighth embodimentof the present invention will be described by using FIG. 16A and FIG.16B. FIG. 16A is a diagram illustrating a configuration example of theimaging system according to the present embodiment. FIG. 16B is adiagram illustrating a configuration example of the movable objectaccording to the present embodiment.

FIG. 16A illustrates an example of the imaging system regarding anon-vehicle camera. The imaging system 500 includes an imaging device510. The imaging device 510 is any of the imaging devices 100 describedin the above first to sixth embodiments. The imaging system 500 includesan image processing unit 512 that performs image processing on aplurality of image data acquired by the imaging device 510 and aparallax calculation unit 514 that calculates a parallax (a phasedifference of parallax images) from the plurality of image data acquiredby the imaging system 500. Further, the imaging system 500 includes adistance measurement unit 516 that calculates a distance to the objectbased on the calculated parallax and a collision determination unit 518that determines whether or not there is a collision possibility based onthe calculated distance. Here, the parallax calculation unit 514 and thedistance measurement unit 516 are an example of a distance informationacquisition unit that acquires distance information on the distance tothe object. That is, the distance information is information on aparallax, a defocus amount, a distance to an object, or the like. Thecollision determination unit 518 may use any of the distance informationto determine the collision possibility. The distance informationacquisition unit may be implemented by dedicatedly designed hardware ormay be implemented by a software module. Further, the distanceinformation acquisition unit may be implemented by a Field ProgrammableGate Array (FPGA), an Application Specific Integrated Circuit (ASIC), orthe like, or may be implemented by combination thereof.

The imaging system 500 is connected to the vehicle informationacquisition device 520 and can acquire vehicle information such as avehicle speed, a yaw rate, a steering angle, or the like. Further, theimaging system 500 is connected with a control ECU 530, which is acontrol device that outputs a control signal for causing a vehicle togenerate braking force based on a determination result by the collisiondetermination unit 518. Further, the imaging system 500 is connectedwith an alert device 540 that issues an alert to the driver based on adetermination result by the collision determination unit 518. Forexample, when the collision probability is high as the determinationresult of the collision determination unit 518, the control ECU 530performs vehicle control to avoid a collision or reduce damage byapplying a brake, pushing back an accelerator, suppressing engine power,or the like. The alert device 540 alerts a user by sounding an alertsuch as a sound, displaying alert information on a display of a carnavigation system or the like, providing vibration to a seat belt or asteering wheel, or the like.

In the present embodiment, an area around a vehicle, for example, afront area or a rear area is captured by using the imaging system 500.FIG. 16B illustrates the imaging system in a case of capturing a frontarea of a vehicle (a capturing area 550). The vehicle informationacquisition device 520 transmits instructions to the imaging system 500or the imaging device 510. Such a configuration can further improve theranging accuracy.

Although the example of control for avoiding a collision to anothervehicle has been illustrated in the above description, the embodiment isapplicable to automatic driving control for following another vehicle,automatic driving control for not going out of a traffic lane, or thelike. Furthermore, the imaging system 500 is not limited to a vehiclesuch as the subject vehicle, and can be applied to a movable object(moving apparatus) such as a ship, an airplane, or an industrial robot,for example. In addition, the imaging system 500 can be widely appliedto any device which utilizes object recognition, such as an intelligenttransportation system (ITS), without being limited to movable objects.

Modified Embodiments

The present invention is not limited to the above-described embodiments,but various modifications are possible.

For example, the embodiment of the present invention includes an examplein which a part of the configuration of any of the embodiments is addedto another embodiment and an example in which a part of theconfiguration of any of the embodiments is replaced with a part of theconfiguration of another embodiment.

Further, the avalanche photodiode illustrated in FIG. 4 is an example,and the structure of the avalanche photodiode applicable to the presentinvention is not limited to that illustrated in FIG. 4. Further, thecharacteristic of the avalanche photodiode is not limited to thatillustrated in FIG. 5. Each voltage of the power source that drives theavalanche photodiode has to be selected as appropriate in accordancewith the characteristic of the avalanche photodiode.

Further, while a plurality of avalanche photodiodes APD in each onepixel are arranged within each one n-type semiconductor region 224 inthe embodiments described above, each of the avalanche photodiodes APDmay be arranged within a separate n-type semiconductor region 224.

Further, the imaging systems illustrated in the seventh and eighthembodiments are illustrated as an exemplary imaging system to which theimaging device of the present invention can be applied, and imagingsystems to which the imaging device of the present invention can beapplied are not limited to the configurations illustrated in FIG. 15 toFIG. 16B.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2017-052956, filed Mar. 17, 2017, which is hereby incorporated byreference herein in its entirety.

1. An imaging device comprising: a plurality of pixels each including aplurality of avalanche photodiodes, a setting unit configured toseparately set the plurality of avalanche photodiodes to an active stateor an inactive state, and a counter circuit that counts and outputsnumber of photons determined by the avalanche photodiode(s) set to theactive state out of the plurality of avalanche photodiodes, wherein theimaging device is configured to change the number of avalanchephotodiodes set to the active state out of the plurality of avalanchephotodiodes, in accordance with brightness of an object. 2.-16.(canceled)